linux-kernel - Re: msync(2) bug(?), returns AOP_WRITEPAGE

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Date:	Sun, 14 Oct 2007 18:32:08 -0400
From:	Erez Zadok <ezk@...sunysb.edu>
To:	Pekka J Enberg <penberg@...helsinki.fi>
Cc:	Erez Zadok <ezk@...sunysb.edu>, Hugh Dickins <hugh@...itas.com>,
	Ryan Finnie <ryan@...nie.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	cjwatson@...ntu.com, linux-mm@...ck.org
Subject: Re: msync(2) bug(?), returns AOP_WRITEPAGE_ACTIVATE to userland 

In message <Pine.LNX.4.64.0710142049000.13119@...-30.cs.Helsinki.FI>, Pekka J Enberg writes:
> Hi Erez,
> 
> On Sun, 14 Oct 2007, Erez Zadok wrote:
> > In unionfs_writepage() I tried to emulate as best possible what the lower
> > f/s will have returned to the VFS.  Since tmpfs's ->writepage can return
> > AOP_WRITEPAGE_ACTIVATE and re-mark its page as dirty, I did the same in
> > unionfs: mark again my page as dirty, and return AOP_WRITEPAGE_ACTIVATE.
> > 
> > Should I be doing something different when unionfs stacks on top of tmpfs?
> > (BTW, this is probably also relevant to ecryptfs.)
> 
> Look at mm/filemap.c:__filemap_fdatawrite_range(). You shouldn't be 
> calling unionfs_writepage() _at all_ if the lower mapping has 
> BDI_CAP_NO_WRITEBACK capability set. Perhaps something like the totally 
> untested patch below?
> 
> 				Pekka
[...]

Pekka, with a small change to your patch (to handle time-based cache
coherency), your patch worked well and passed all my tests.  Thanks.

So now I wonder if we still need the patch to prevent AOP_WRITEPAGE_ACTIVATE
from being returned to userland.  I guess we still need it, b/c even with
your patch, generic_writepages() can return AOP_WRITEPAGE_ACTIVATE back to
the VFS and we need to ensure that doesn't "leak" outside the kernel.

Erez.
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